There is a great need for and interest in more efficient means of converting chemical energy into electrical energy, which has created great interest in fuel cells. The practical commercial development of these has, though, been held back by numerous practical problems. The SOFC is a particularly attractive system, which can utilize hydrocarbon fuels such as methane with internal reforming of the fuel, and achieve relatively high efficiencies. Significant problems remain, though, in relation to inter alia the design of the anode used.
Many different materials have been tried in the search for improved anode performance, including materials such as Ni/YSZ (YSZ=Y2O3/ZrO2) which has relatively good performance but suffers from the substantial disadvantage of relatively short working life due to the formation of carbon deposits when using hydrocarbon fuels, susceptibility to poisoning with sulphur which is frequently encountered in hydrocarbon fuels and an intolerance to repeated reduction/oxidation cycling, as was encountered in small systems such as CHP units or APUs for automotive applications. It has also been proposed to use LaCrO3 (P. Vernoux et al J. Electrochem. Soc. 145 3487-3492 (1998)), and more recently it has been proposed to utilize LaCrO3 which has been doped with various elements in order to improve its performance (J. Liu et al Electrochemical and Solid-State Letters 5 A122-A125 (2002)). Nevertheless even such doped LaCrO3 has relatively limited electrochemical performance and effective working life.
It is an object of the present invention to avoid or minimize one or more of the above mentioned disadvantages.
It has now, by consideration of a novel approach, been found that by using a double perovskite material based on LaCrO3 instead of a doped LaCrO3, it is possible to achieve electrical and catalytic properties comparable with prior art anode materials such as Ni based anodes (that is with over-potential losses which can be less than 100 mV at current densities of 400 mAcm−2) without the need for using metallic current conducting components, normally nickel and without significant carbon formation and deposition when using hydrocarbon fuels. Unlike previously tried doped LaCrO3 in which a small number of the La and/or Cr atoms in LaCrO3, typically 5 to 10%, or at most 20%, are replaced by different atoms, resulting in a doped form of a “single” perovskite, in a double perovskite material the B sites of the perovskite crystal lattice structure, normally occupied substantially wholly by Cr, are occupied by similar amounts of two different elements. It should be emphasised that the term double perovskite is used here to emphasise double occupation B-site, and not necessarily to imply any structural order that manifests itself as two different structural B-sites existing in the lattice.